Electricity meter registers monitor line voltage and current as a function of time at each customer site to obtain electrical energy consumption billing data. This generally is carried out by electromechanical watt hour meters that use a disc which rotates as a function of electrical energy consumption and contain registers that display the electrical energy accumulated during a predetermined period of time, e.g., within one billing month. The meters must be read and reset at the end of each month.
More sophisticated electrical energy consumption registers measure both kilowatt hour consumption and kilowatt demand at different billing rates depending upon the "real time" of consumption. "Real time" as used herein refers to time measured in terms of day of week and hours, minutes and seconds of the day. Customer billing rates are determined by the energy supplier in accordance with predetermined time or demand intervals. For example, electrical energy consumption measured during predetermined intervals of peak demand for energy are billed at a highest billing rate, and energy consumption at other predetermined time intervals of the day are billed at correspondingly lower billing rates. This method of metering is known as "time-of-day" or "time-of-use" metering and is commonly carried out in the industry to unify energy consumption throughout the day.
In time-of-use metering apparatus using solid-state circuitry to develop real time-of-use data to be stored in a solid state memory, a microprocessor-based control interrogates the output of the rotating electromechanical disc or line voltage and current measurement sensors to develop line power which is correlated with predetermined real time-of-use intervals to obtain customer billing data. An important requirement of such equipment is to retain the accumulated electrical energy consumption and real time data during a power outage, so that upon resumption of line power, no customer billing data or time data are lost. In this regard, time must continue to be measured during the power outage; otherwise, upon resumption of power, energy consumption will not be properly correlated with time-of-use intervals to develop accurate customer billing data.
An example of a solid-state time-of-use electricity meter register, described in Johnston et al U.S. Pat. No. 4,197,582, has a microprocessor based circuit for interrogating line voltage and current measurement sensors to obtain electrical energy consumption data, and a real time clock in the microprocessor correlates the electrical energy consumption data with predetermined time-of-use intervals, to develop customer billing data. During normal operation of the register, the circuit is powered by a main DC power supply connected to the power lines. Upon a power outage, the line power and real time data accumulated by the internal real time clock are written into an external random access memory that is powered by a backup battery. An external counter, also powered by the backup battery, increments in response to pulses generated at a repetition rate of one pulse per eight seconds during the interval of the line power outage. Upon resumption of line power, the electrical energy consumption and real time data previously stored in the external memory are transferred back to the microprocessor. The count stored in the external counter is transferred to the microprocessor wherein the count is multiplied by the period of the counter (eight seconds), and the result is applied to update the real time read from the external memory. Updating of the real time of the microprocessor by the count stored in the counter is initiated upon the first eight second increment following resumption of line power.
The pulse counting technique used in the Johnston et al patent presents a severe limitation to the maintenance of accurate customer billing data during a power outage, because updating of the real time maintained by the internal clock occurs up to eight seconds after the resumption of line power. Accordingly, among a large number of such meter registers within a region affected by a power outage, there is, on the average, a four second delay following resumption of power before the internal real time clock of each register is updated. Until the internal real time clock is updated, the energy consumed by each customer cannot be billed because there is no association between the energy consumed during the up to eight second delay and its corresponding real time-of-use billing interval. This represents a significant source of inaccuracy because there is substantial electrical power consumption by demand loads immediately upon resumption of power. Loss of energy information associated with each register must be multiplied by the number of registers, e.g. thousands of registers, within a particular distribution system affected by a power outage.
Accordingly, one object of this invention is to provide a method of and system for retaining electrical power and real time data within a time-of-use meter register during power outages.
Another more particular object is to minimize loss of power and time-of-use data during power outages, by updating time-of-use data immediately following power resumption.
A further object is to provide a method of and system for providing a battery backup for a solid-state electricity meter register wherein stored time-of-use data are updated by the elapsed time of the outage immediately upon resumption of line power.
As another disadvantage of the Johnston et al apparatus, because the external memory for storing electrical power and real time data is powered by the back-up battery and is not, itself, non-volatile, current drain from the battery is relatively large, and will maintain the memory non-volatile for only a somewhat limited period of time. Furthermore, in the event that the back-up battery fails or fully discharges during the interval of the power outage, all the energy consumption and real time data previously stored in the memory at the time the outage occurs are lost.
Another object of the invention, therefore, is to provide a battery back-up system in a solid-state time-of-use electricity meter wherein battery current drain is minimized.
A further object is to provide a fail-safe system in the battery back-up circuit of a solid state time-of-use electricity meter register wherein the electrical energy consumption and time-of-use data stored at the time of the power outage are not lost in an event of a back-up battery failure.
Another object is to prevent loss of electrical energy consumption data in a solid-state time-of-use electricity meter register upon a power outage and back-up battery failure.